1. Field of the Invention
The present invention relates to an injection apparatus and a method of controlling the same.
2. Description of the Related Art
Conventionally, an injection apparatus is disposed in an injection molding machine. A screw is disposed within a heating cylinder of the injection apparatus such that the screw can be rotated and reciprocated by a metering motor and an injection motor. A spiral flight is formed on the outer circumferential surface of the screw, so that a groove is formed by the flight.
When the screw is rotated in a forward direction during a metering step, pellets of resin which have fallen from a hopper are melted, and molten resin is caused to advance along the groove. As a result, the molten resin is accumulated on the front side of a screw head, and the screw is retracted accordingly.
In order to enable the above-mentioned metering operation, a resin supply section to which pellets of resin are supplied from the hopper, a compression section in which the supplied resin is melted while being compressed, and a metering section in which a predetermined amount of the molten resin is measured are formed on the screw, in this order from the rear end toward the front end. The resin in the groove assumes the form of pellets in the resin supply section, comes into a semi-molten state in the compression section, and is melted completely into liquid in the metering section. The resin supply section, the compression section, and the metering section constitute a flight portion. In the case in which the outer circumferential surface of the screw and the inner circumferential surface of the heating cylinder are of equal surface roughness, when the screw is rotated during the metering step, the resin within the groove rotates together with the screw and does not advance. In order to avoid this problem, in general, the inner circumferential surface of the heating cylinder is made rougher than the outer circumferential surface of the screw.
When the screw is advanced during an injection step, the resin accumulated in front of the screw head is injected from an injection nozzle and charged into a cavity of a mold unit. In order to prevent backflow of the resin accumulated in front of the screw head, which backflow would otherwise occur during the injection step, a backflow prevention unit is disposed around the screw head. The backflow prevention unit includes an annular backflow prevention ring which is disposed to surround a shaft portion of the screw head, and a press metal plate fixed to the front end of the flight portion of the screw. Upon rotation of the screw, the backflow prevention ring assumes a communication position at which the space on the front side of the screw head communicates with the flight portion, and a shut-off position at which the communication between the space on the front side of the screw head and the flight portion is broken. Accordingly, when the screw is rotated in the reverse direction after completion of the metering step to thereby move the backflow prevention ring to the shut-off position, backflow of the resin accumulated in front of the screw head is prevented.
Incidentally, if the pressure of resin in front of the screw head upon starting of the injection step; i.e., injection pressure, involves variation, sink marks and burrs are formed on molded products, resulting in deterioration in quality of the molded products. In order to overcome this problem, the following operation sequence is employed. After completion of the metering step, the backflow prevention ring is moved to the shut-off position, and the screw is temporarily advanced. When the injection pressure reaches a preset pressure, advancement of the screw is stopped, and the stopped position of the screw is stored in a memory as a reference position. Subsequently, the screw is retracted from the reference position so as to perform a suck-back operation. Subsequently, the injection step is started in order to advance the screw by a preset distance from the reference position. The preset distance is calculated on the basis of a quantity of resin to be charged into the cavity, and a distance over which the screw is retracted during the suck-back operation.
As described above, during the injection step, the screw is advanced by a preset distance from the reference position, so that the injection pressure can be maintained at the preset pressure at all times. Therefore, formation of sink marks and burrs on molded products is prevented, and thus the molded products are of improved quality.
However, the conventional injection apparatus has the following drawbacks. Since the inner circumferential surface of the heating cylinder is made rougher than the outer circumferential surface of the screw, when the screw is advanced, a large frictional resistance acts on the resin in the vicinity of the inner circumferential surface of the heating cylinder. In addition, since the state of the resin in the groove of the screw changes while the resin moves from the resin supply section to the compression section and further to the metering section, the frictional resistance acting on the resin varies accordingly.
Therefore, in the injection step, the injection pressure does not correspond to an injection force applied to the screw from its rear end, with the result that the resin cannot be injected with a sufficiently high injection pressure, and the injection pressure varies with the frictional resistance.
As a result, the pressure of resin within a mold; i.e., the mold inner pressure, involves variation, with resultant deterioration in quality of molded products.
Moreover, when the screw is advanced to the reference position after completion of the metering step, a large frictional resistance acts on the resin in the vicinity of the inner circumferential surface of the heating cylinder, so that the relationship between the injection pressure and the position of the screw changes. Therefore, even when the screw is stopped when the injection pressure reaches the preset pressure, the stopped position of the screw varies. As a result, the reference position varies, which lowers the quality of molded products.